Vehicles, which have a hybrid drive structure, have an internal combustion engine and in most cases an electric motor as the second power plant. The driving torque may thus be generated by both power plants during the driving operation of the hybrid vehicle.
In order to maintain the minimum rotational speed, internal combustion engines have an idle speed controller which compares a setpoint rotational speed with an actual rotational speed and generates a controller torque from this comparison, which sets the setpoint rotational speed. The control includes intensification factors which are adapted to the particular engine in an application operation. According to the state of the particular controlled system, which is formed by the drivetrain, the engine, and the vehicle, different intensification factors, which are available as parameter sets, are selected depending on whether the internal combustion engine is warm or cold or is operated in the first or a different gear. Higher intensification factors improve the dynamics of the idle speed controller. However, if they are too high, oscillations of the controller output signal may occur, thus resulting in an instability of the control.
In hybrid vehicles, the electric motor has significantly higher dynamics when building up and reducing the torque, which is output by the electric motor, than the internal combustion engine. However, it must be taken into account that for idle speed control the electric motor outputs only a limited torque or none at all, which is, for example, due to the fact that the high voltage battery supplying the electric motor with energy is empty, or overheating of the electric motor takes place. Therefore, the control intensification factors in the idle speed controller of the hybrid vehicle are selected to be only high enough to achieve a stable idle speed without the support by the electric motor. Adding other parameter sets which indicate during the idle speed control of the internal combustion engine that the electric motor has failed, for example “cold internal combustion engine with support of the electric motor,” or “cold internal combustion engine without the support of the electric motor,” would double the parameter sets of the internal combustion engine and is therefore very complex.